1. Field of the Invention
The present invention relates to a hole transporting compound for organic electroluminescent devices, and to a method for producing the same. More particularly, the present invention relates to a hole transporting compound with a good thermal stability having 6,6xe2x80x2-bis(9H,9-carbazolyl)-N,Nxe2x80x2-disubstituted-3,3xe2x80x2-bicarbazyl as a basic molecular structure, and to a method for its production.
2. Description of Background Art
Electronic technology has improved human life since silicon was first used as a semiconductor material. The fast growth of photocommunications and multimedia in particular has accelerated the formation of a highly information-oriented society. Consequently, optoelectronic devices (OEDs) based on the conversion of photons into electrons or vice versa are very important elements in the electronic information industry nowadays. Semiconductive optoelectronic devices can be classified as electroluminescent devices, light-receiving type devices and combined devices.
Most display means are of the light-receiving type, whereas an electroluminescent display (ELD) is a self-luminescent type device. Since the ELD has the advantages of fast responsiveness, obviation of backlight, excellent luminance, as well as wide viewing angle, ELDs have been studied for many applications including natural color display devices. These electroluminescent devices have been developed and practically used in inorganic semiconductors which employ GaN, ZnS and SiC as display devices.
However, an EL device prepared from an inorganic material needs more than 220V for the driving voltage. Furthermore since the preparation of the EL device is carried out by means of a vacuum deposition, a large size device cannot be prepared and the cost of preparation is very high. Thus, not only EL devices using inorganic materials, but also EL devices using organic materials have been developed. Pope et al. disclosed electroluminescence of organic materials in 1963. In 1987, Tang et al. of the Eastmann Kodak company presented a green-emitting device, namely a multi-layer electroluminescent device with 1000 cd/m2 of luminance, and 1% of quantum efficiency at less than 10V, which was prepared by using a pigment with a Π-conjugated structure, called xe2x80x9ctris(8-hydroxyquinoline) aluminum: Alq3xe2x80x9d. Thereafter, research on EL devices using organic materials has been actively pursued.
In particular, low-molecular-weight organic materials for ELDs have advantages such as simple synthetic methods and flexibility of molecular structure via appropriate molecular design. Displays using low-molecular-weight organic materials have been developed in sizes up to 10 inches for fall color displays of the passive matrix type and 3 inches for full color displays of the active matrix type using thin film transistors.
However, the major problems with OEDs are light-emitting efficiency and lifetime. Improved lifetimes of 20,000 hours in blue (Indemitsu), 50,000 hours in green (Eastman Kodak), and 20,000 hours in red (Eastman Kodak) respectively, have been achieved, but the light-emitting efficiency of red and blue requires further improvement. Therefore, there are still some problems remaining before applications in fall color display are beneficial.
Furthermore, it is impossible to achieve a high efficiency and luminance without using a multilayer system which consists of a buffer layer, a hole transporting layer, an electron transporting layer and a hole blocking layer for light-emitting efficiency. In order to achieve high efficiency and luminance in a device, it is necessary for each functional layer to have thermal or electrical stability. In particular, the hole transporting layer material requires thermal and electrical stability because the stability critically affects the lifetime of the device. That is, the molecules with low thermal stability are believed to have low morphological stability, which brings about morphological changes in the organic layers used in the device when voltage is applied. Furthermore, the change may initiate partial crystallization to decrease the light-emitting efficiency, resulting in a reduction in the lifetime of the device.
The glass transition temperature of the TPD [N,Nxe2x80x2-bis(3-methylphenyl)-N,Nxe2x80x2-diphenyl-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diamine] or NPB [N,Nxe2x80x2-bis(naphthalene-1-yl)-N,Nxe2x80x2-diphenylbenzidine, NPD] frequently used these days are 60xc2x0 C. and 96xc2x0 C. respectively, which are not high temperatures. The vacuum deposition layer of the hole transporting layer, which forms a uniform amorphous film, becomes crystallized or aggregated due to the loss of film uniformity, which results in a reduction in the lifetime of the device. It is necessary for the hole transporting layer to have a Tg of more than 100xc2x0 C. and an operating or storage temperature of 85xc2x0 C. in order to use the device outdoors or in vehicles. Thus it is important to further improve the thermal stability and high glass transition temperature of hole transporting materials.
Accordingly, the present inventors have developed a hole transporting compound with good thermal stability and high glass transition temperature having 6,6xe2x80x2-bis(9H,9-carbazolyl)-N,Nxe2x80x2-disubstituted-3,3xe2x80x2-bicarbazyl as a basic molecular structure, and methods for producing the same, in order to overcome these problems.
A feature of the present invention is the provision of a hole transporting compound for organic electroluminescent devices with high thermal stability, and a method for producing the same
Another feature of the present invention is the provision of a hole transporting compound for organic electroluminescent devices with a high glass transition temperature, and methods for producing the same.
A further feature of the present invention is the provision of a hole transporting compound for organic electroluminescent devices having good electrical stability, and methods for producing the same.
A further feature of the present invention is the provision of an organic electroluminescent device with high luminescence efficiency.
A further feature of the present invention is the provision of an organic electroluminescent device having an extended lifetime.
In accordance with one aspect of the present invention, there is provided a hole transporting compound having the following formula (I) including 6,6xe2x80x2-bis(9H,9-carbazolyl)-N,Nxe2x80x2-disubstituted-3,3xe2x80x2-bicarbazyl as a basic molecular structure: 
wherein R is a hydrogen atom, a C1-12 aliphatic alkyl group, a C3-12 branched alkyl group, a C5-12 cyclic alkyl group, or a C4-14 aromatic group, more particularly a C6-14 aromatic group, wherein the aromatic group can have one or more alkoxy or amine substituents.